Niddah, Chapter 6, Mishnah 10. This chapter is built out of a long series of general rules, each one phrased in the same way: whatever has this feature also has that feature, but the reverse is not always true. The Mishnah has been applying that pattern to questions of tumah and taharah, and here it turns to two well known simanim of kashrus, the signs of a kosher fish and the signs of a kosher animal.
The first of the two rules speaks about fish. "Kol sheyesh lo kaskeses", any fish that carries a scale, "yesh lo snapir", will be found to possess a fin as well. The reverse, though, does not hold true: "V'yesh sheyesh lo snapir", there are fish equipped with a fin, "v'ein lo kaskeses", upon which no scale is found at all.
The background to this is the siman by which we identify a kosher fish: it must have both a fin and a scale. Now, since scales never appear on their own, a fish found with scales is automatically known to have fins as well, and it is kosher. Fins, however, tell us nothing on their own, because plenty of fish carry fins with no scales at all. The sign works in one direction only.
The Mishnah then applies the identical pattern to land animals: "Kol sheyesh lo karnayim, yesh lo tlafayim", every animal that has horns also has split hooves. "V'yesh sheyesh lo tlafayim v'ein lo karnayim", and there are animals with split hooves that have no horns.
Here too the practical value of the rule lies in what we are looking for. What matters to us is the split hoof, one of the simanim of a kosher animal. The Mishnah teaches that horns are a reliable indicator: an animal that carries horns certainly has split hooves as well. But the indicator cannot be reversed. A split hoof does not testify to horns, since there are animals whose hooves are split and whose heads carry no horns whatsoever.
In both halves of the Mishnah, then, one feature guarantees the other while the other guarantees nothing, and that one sided relationship is precisely the shape of every rule in this chapter.